qobject: Clean up around qtype_code

QTYPE_NONE is a sentinel value.  No QObject has this type code.
Document it properly.

Fix dump_qobject() to abort() on QTYPE_NONE, just like for any other
invalid type code.

Fix to_json() to abort() on all invalid type codes, not just
QTYPE_MAX.

Clean up Property member qtype's type: it's a qtype_code.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

block/qapi.c

@@ -523,9 +523,6 @@ static void dump_qobject(fprintf_function func_fprintf, void *f,
             QDECREF(value);
             break;
         }
-        case QTYPE_NONE:
-            break;
-        case QTYPE_MAX:
         default:
             abort();
     }